Subscriber line driver and termination

ABSTRACT

Various embodiments are configured to transform characteristics of a communication signal. One embodiemnt is a method comprising decreasing amplitude of a first detected portion of the communication signal so that the decreased amplitude is in close proximity to a predefined specification; and increasing amplitude of a second portion of the communication signal so that the increased amplitude is in close proximity to the predefined specification, thereby resulting in a transformed communication signal

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application is a divisional of co-pending U.S. utilityapplication entitled, “SUBSCRIBER LINE DRIVER AND TERMINATION,” havingSer. No. 09/439,933, filed Nov. 12, 1999, which is entirely incorporatedherein by reference.

[0002] This document claims priority to and the benefit of the filingdate of co-pending commonly assigned Provisional Application entitled,“SUBSCRIBER LINE DRIVER AND TERMINATION,” having Ser. No. 60/108,044,filed Nov. 12, 1998. The foregoing pending provisional application ishereby incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

[0003] The present invention relates generally to the art of datacommunications. The preferred embodiment generally relates to the art oftelephony, and more particularly, to a communication signal driversystem (and associated methodology) for connection between a telephonyswitching unit, which may be located at a central office (CO), at aprivate branch exchange (PBX) or the like, and customer premisesequipment via an existing telephony connection (e.g., copper wiretwisted-pair, digital subscriber loop or the like).

BACKGROUND OF THE INVENTION

[0004] With the increasing bandwidth demands from the advent of theInternet, service providers have looked for ways to increase dataperformance over the copper wire twisted-pair local loop transmissionlines that connect the telephone central offices (COs) to the customerpremises (CPs). The customer premises equipment (CPE) is connected tothe CO switches over transmission lines known as “local loops,”“subscriber loops,” “loops,” or the “last mile” of the telephonenetwork. Historically, the public switched telephone network (PSTN)evolved with subscriber loops connected to a telephone network withcircuit-switched capabilities that were designed to carry analog voicecommunications. Digital service provision to the customer premises is amore recent development, with the evolution of the telephone networkfrom a system just designed to carry analog voice communications into asystem which could simultaneously carry voice and digital data.

[0005] Because of the prohibitive costs of replacing or supplementingexisting subscriber loops, technologies have been implemented thatutilize existing subscriber loops to provide easy and low cost customermigration to digital technologies. Subscriber loops capable of carryingdigital channels are known as digital subscriber lines (DSLs). Logicalchannels within a subscriber line which carry digital signals are knownas DSL channels, while logical channels within a subscriber line whichcarry plain old telephone service (POTS) analog signals are known asPOTS channels. Furthermore, to provide customers with additionalflexibility and enhanced services, frequency-division multiplexingand/or time-division multiplexing techniques have been designed to filla subscriber loop with multiple logical channels. These newer DSLtechnologies provide digital service to the customer premises withoutsignificantly interfering with the existing POTS equipment and wiring.The newer DSL technologies accomplish this functionality byfrequency-division multiplexing (FDM) their digital signal above (athigher frequencies than) the 0 KHz to 4 KHz baseband of standard, analogPOTS signals. Multiplexing techniques and terminology are common tothose skilled in the art, and are not described herein.

[0006] Several variants of new DSL technology exist (e.g., ADSL, SDSL,RADSL, VADSL, MVL™, Tripleplay™, etc.), with this group generallyreferred to as xDSL. Communications systems carrying xDSL usuallymultiplex xDSL signals and a POTS signal onto a single physical localloop.

[0007] Historically, the POTS subscriber loop was designed with thefunctions needed to communicate both analog, voice-conversation signalsand subscriber loop signaling. The CO switch uses subscriber loopsignaling to notify the customer premises about events in the telephonenetwork, while customer premises equipment (CPE) use subscriber loopsignaling to inform the CO to perform actions for the customer. Someexamples of subscriber loop signaling include: the CO switch signalingto the CPE that an incoming call has arrived by ringing the phone, theCPE (e.g., a telephone) signaling to the CO switch that the CPE isinitiating a call by an on-hook to off-hook transition of the telephonehandset, and the CPE signaling to the CO switch that a call should beconnected to a location by sending the phone number of the location.

[0008] Although the transmission of both digital and analog POTS signalsover a subscriber loop offers many potential advantages for customers,several practical problems must be solved in implementing DSL solutions.One significant problem resulting from the POTS subscriber loopsignaling functions is the generation of high-frequency interference,known in the art as noise, into DSL channels. For instance, theon-hook/off-hook signal and the pulse-dialing signal are squarewaveforms which have high-frequency components and harmonics, andtheoretically require infinite frequency bandwidth. This high-frequencynoise may degrade the signal to noise (S/N) ratio of the DSL channel.The S/N ratio is commonly known to those skilled in the art, but can besimply described as the ratio of the transmit signal amplitude to thenoise amplitude, expressed in decibels (dB). Thus, a heretoforeunaddressed need exists in the industry for a way to prevent orsubstantially minimize the adverse affects on the DSL channel S/N ratiocaused by noise introduced by the POTS subscriber loop functions.

[0009] Another practical problem facing the industry effort to implementDSL technology on the existing PSTN system is the large voltagemagnitude change occurring on the subscriber loop during transitionsbetween on-hook and off-hook conditions, as is well known in the art.Some embodiments of prior art DSL technology require a change in theinput impedance of the DSL device upon sensing of a transition betweenon-hook and off-hook conditions. Thus, a heretofore unaddressed needexists in the industry for a way to prevent or substantially minimizethe adverse affects of the on-hook/off-hook transition.

[0010] Another practical problem facing the industry effort to implementDSL technology on the existing PSTN system is the unpredictable natureof the subscriber loop transmission system impedance. Signal attenuation(decrease in signal strength) and signal distortion (changes in thesignal shape) are caused by real and reactive impedance losses incurredon the subscriber loop as the signal is transmitted between the CO andthe CPE. Each subscriber loop, consisting of a copper wire twisted-paircircuit connecting the CO to the CPE, is unique. That is, eachsubscriber loop differs in length, and often these subscriber loops areconstructed with varying copper wire gauge sizes. Therefore, the actualcircuit impedance of any given subscriber loop is unique and differentfrom other subscriber loops. DSL technology utilizes FDM to shift thefrequency of the communication signal into the 25 KHz to 1 MHz frequencyrange. As is well known in the art, subscriber loop circuit impedance isnot a constant, but rather a variable over the frequency spectrumbecause the subscriber loop impedance is complex (having reactiveimpedance components as well as resistive impedance components).Therefore, signal attenuation also varies with the frequency of atransmission signal. That is, some frequencies will be attenuated moreor less than other frequencies.

[0011] The presence of bridged taps connected to the subscriber loopintroduces another unpredictable impedance component. Bridged taps areunused copper wire twisted-pair lengths connected at various points ofthe subscriber loop. Bridged taps constitute parallel circuits whichalter the impedance of the subscriber loop circuit, and effectivelyreduce the transmit signal strength.

[0012] Finally, the wiring of the customer premise and the various typesof customer equipment and devices, including multipoint communication,connected to the subscriber loop is unique. These differences at thecustomer premise also impact the overall impedance of the subscriberloop transmission system.

[0013] For the purpose of establishing the transmitter frequency domainspecifications and limits, current practice typically models thesubscriber loop impedance as a resister, RL, that is representative ofthe characteristic impedance of the subscriber loop transmission line.At the remote end of the transmission line, the receiver equipment istypically modeled as a terminating resister, R_(R), usually of the samevalue as R_(L). Transmission of signals onto subscriber loops has beenprovided by a voltage signal source, V_(s), and a series resister,R_(T). Current practice is to transmit at the subscriber looptransmission line input a transmit signal spectral shape of V_(S) thatis designed to be the same as a voltage power spectral distribution(PSD) standard. The PSD standard specifies maximum signal strength(amplitude) and frequency bandwidth boundaries for a DSL channel.

[0014] Design of the transmit signal spectral shape of V_(S) necessarilyrequires certain assumptions about the subscriber loop transmissionsystem. Traditional transmission line theory teaches that for optimumcommunication, the subscriber loop transmission system should haveR_(T)=R_(L)=R_(R). As an example, it is customary in some DSLtechnologies to select R_(L)=135 ohms for transmission signals in theband from approximately DC to 192 kHz. This 135 ohm value is areasonable best choice for a simplistic resistive compromise model.Because the prior art model is resistive, the design transmit signal isthe same as the design PSD of V_(S).

[0015] However, the prior art assumptions may be wholly inadequate inrepresenting the wide range of subscriber loop transmission lines foundin practice. R_(T) is not ideal (R_(T)≠R_(L)≠R_(R)) since eachindividual subscriber loop is unique. Also, R_(L) is not ideal becausecustomer premises wiring are often different and because of bridged tapson the subscriber loop. In practice, the actual subscriber looptransmission system impedance can vary in magnitude from well over 200ohms to less than 50 ohms, and the actual impedance is complex. Theresult in practice is that the actual transmit signal on any giventransmission line can vary dramatically, and this variance is usuallysuch that the transmit signal amplitude is lower than permitted in partof or all of the transmission band as defined by the PSD standard. Itcan be shown, for example, that the actual transmit signal amplitude canbe 12 dB lower than the PSD standard in part of the band, and evenaverage power can be 6 dB lower than allowed. This means that 6 dB ormore of potential transmit signal power is being sacrificed, and thatthe receive signal S/N ratio is thus 6 dB lower than the S/N that couldbe realized with an optimized transmit signal.

[0016] Another problem involves instances where the actual transmitsignal voltage exceeds the PSD standard. If the actual transmit signalvoltage exceeds the PSD standard, undesirable interference or noise isinduced onto other subscriber loops sharing the same underground cableor overhead wire.

[0017] Thus, a heretofore unaddressed need exists in the industry for away to provide for a transmit signal which conforms to a defined PSDstandard regardless of the actual impedance characteristics of thetransmission system.

SUMMARY OF THE INVENTION

[0018] The present invention provides a subscriber line driver (SLD) fortransforming the characteristics of a communication system signal. Thesignal is transformed by the SLD which increases (amplifies) portions ofthe signal to a predefined specification, decreases (attenuates)portions of the signal to a predefined specification, and/or frequencymodulates or filters the transmit signal frequencies to fit within thecommunication channel frequency bandwidth as defined by the frequencyband of the predefined specification. After modification by the SLD, thetransformed communication signal is injected (transmitted) into acommunication transmission line. The SLD may operate in a continuous andautomatic mode. An SLD may be applicable to a variety of communicationsystems, for example but not limited to, a public telephony system, aprivate branch exchange (PBXs), a coaxial cable system, a fiber opticsystem, a microwave system or a radio communication system. In thepreferred embodiment, the SLD operates on a telephony system subscriberloop which is operated as a digital subscriber loop (DSL) having a plainold telephone system (POTS) channel and at least one DSL channel.

[0019] The method of the preferred embodiment of the SLD comprises thefollowing steps. The direction of travel of a communication signal issensed when in the transmit signal direction, where the transmitdirection is defined as traveling in a direction out to thecommunication system, here a subscriber loop. The SLD transforms thecommunication signal traveling in the transmit direction such that thetransformed communication signal conforms to a predefined specification.

[0020] The preferred embodiment of the SLD comprises at least twofunctional components, a transmit signal equalizer and a current driverconnected to the output of the transmit signal equalizer. In thepreferred embodiment, the current driver injects (transmits) thetransformed communication signal into the subscriber loop. Anotherembodiment of the SLD utilizes a voltage driver (rather than the currentdriver). A voltage feedback loop can be added to the SLD circuitry whichfurther optimizes the transformed communication signal.

[0021] The SLD has an infinite input impedance at all frequencies.Addition of a parallel resister connected to a tip wire and a ring wireof the telephony system can enable the design engineer to set thetransmission system terminating impedance to any desired value.

[0022] Another embodiment of the SLD modifies the transmit signal toconform to a first predefined specification, and also modifies thereceive signal to conform to a second predefined specification. Thisembodiment of the SLD may have any of the methods, features and optionsof the SLD embodiments previously described.

[0023] This invention also provides for a telephony system centraloffice (CO), comprising at least one telephony switching unit, at leastone digital equipment unit and at least one subscriber line driver(SLD). The telephony switching unit is ultimately connected to atelephony transmission system on one side and to at least one telephonysubscriber loop or DSL on the other side. At least one subscriber linedriver (SLD) would be connected between one terminal of the digitalequipment unit and one subscriber loop or DSL. The SLD would receive acommunication signal from the digital equipment unit, and wouldtransform the communication signal into a transformed communicationsignal so that the transformed communication signal conforms to apredefined specification.

[0024] This invention also provides for a private branch exchange (PBX),comprising at least one telephony switching unit, at least one digitalequipment unit and at least one subscriber line driver (SLD). Thetelephony switching unit is ultimately connected to a telephonytransmission system on one side and to at least one of telephonysubscriber loop or DSL on the other side. At least one SLD would beconnected between one terminal of the digital equipment unit and onesubscriber loop or DSL. The SLD would receive a communication signalfrom the digital equipment unit, and would transform the communicationsignal into a transformed communication signal so that the transformedcommunication signal conforms to a predefined specification.

[0025] Other features and advantages of the present invention willbecome apparent to one skilled in the art upon examination of thefollowing drawings and detailed description. It is intended that allsuch additional features and advantages be included herein within thescope of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] The invention can be better understood with reference to thefollowing drawings. The elements of the drawings are not necessarily toscale relative to each other, emphasis instead being placed upon clearlyillustrating the principles of the present invention. Furthermore, likereference numerals designate corresponding parts throughout the severalviews.

[0027]FIG. 1 is a block diagram of an existing telephony system of theprior art.

[0028]FIG. 2 is a block diagram of an SLD of an embodiment of thepresent invention located on the premises of a transmitting company,organization, and/or individual.

[0029]FIG. 3A is a graph illustrating ideal transmit signal amplitudespectra for a POTS channel and two DSL channels.

[0030]FIG. 3B is a graph illustrating non-ideal transmit signalamplitude spectra for a POTS channel and two DSL channels.

[0031]FIG. 3C is a graph illustrating the modification of a non-idealtransmit signal by the preferred embodiment of the SLD of FIG. 2.

[0032]FIG. 3D is a graph illustrating the modification of a non-idealtransmit signal by another embodiment of the SLD of FIG. 2.

[0033]FIG. 3E is a graph illustrating the modification of a non-idealtransmit signal by the another embodiment of the SLD of FIG. 2.

[0034]FIG. 4 is a block diagram illustrating an SLD located at thetelephone company central office and an SLD located at the customerpremises.

[0035]FIG. 5A is a block diagram illustrating two components of a firstembodiment of the SLD of FIG. 4, a transmit signal equalizer and acurrent driver.

[0036]FIG. 5B is a block diagram illustrating two components of a secondembodiment of the SLD of FIG. 4, a transmit signal equalizer and avoltage driver.

[0037]FIG. 6 is a block diagram illustrating electrical components ofthe SLD of FIG. 4; a transmit signal equalizer, a current driver, anamplifier and a parallel resister (R_(o)).

[0038]FIG. 7A is a block diagram illustrating a central office with aSLD located at the premises of the central office.

[0039]FIG. 7B is a block diagram illustrating a private branch exchange(PBX) with a SLD located at the premises of the PBX.

[0040]FIG. 8 is a diagram illustrating a transmitter with a SLD.

DETAILED DESCRIPTION

[0041]FIG. 1 is a block diagram illustrating an existing telephonysystem 20 which includes a telephone company central office (CO) 22connected to a customer premises (CP) 24 via a subscriber loop 26. Thesubscriber loop 26 may be any suitable connection for passing electricalsignals, but is typically a copper wire twisted-pair, as is well knownin the art, that was originally designed to carry a 0-4 KHz analog voicechannel. Located within the CO 22 is the CO telephony switching unit 28which transmits communication signals received from the outside world tothe CP 24 via the subscriber loop 26, or which receives communicationsignals from the customer premises equipment (CPE) 29 via the subscriberloop 26 for transmission to designated locations in the outside world.In the context of this disclosure describing the existing telephonysystem, “outside world” means any telephone or communications deviceconnected to or having access to the global telephone network, thepublic switched telephone network (PSTN) and/or a private telephonysystem, and where designated locations in the outside world areidentified by telephone numbers or some other identification mannercommonly employed by the art. CO digital equipment 21 and CP digitalequipment 52 may be added at the central office and the customerpremises to facilitate transmission of digital data. When the copperwire twisted-pair is used for digital transmission, the twisted-pair isoften referred to as a digital subscriber loop (DSL). “Central office”or “CO” means any site where a subscriber loop 26 connects into atelephony switching unit, such as a public switched telephone network(PSTN), a private branch exchange (PBX) telephony system, or any otherlocation functionally connecting subscriber loops to a telephonynetwork.

[0042]FIG. 2 is a block diagram illustrating the relative location ofthe preferred embodiment of the subscriber line driver (SLD) at thetransmit signal site. The preferred embodiment of the SLD continuouslyand automatically modifies a non-ideal communication signal amplitudespectra 238, which will be further described in detail hereinafter inFIG. 3A through FIG. 3E, received from the transmit signal equipment128, to fit within the frequency bandwidth and within the maximumamplitude of the PSD standard 40 (FIG. 3A) prior to injecting(transmitting) the transformed communication signal into thecommunication connection 126. The communication signal is then deliveredto the receive signal equipment 129.

[0043] The method of the preferred embodiment of the SLD comprises thefollowing steps. The direction of travel of a communication signal issensed when in the transmit signal direction, where the transmitdirection is defined as traveling in a direction out to the subscriberloop. The SLD transforms the communication signal traveling in thetransmit direction such that the transformed communication signalconforms to a predefined specification or a predefined differencethreshold. This method is described in detail hereinafter.

[0044]FIG. 3A illustrates examples of an ideal communication signalamplitude spectra 32 consisting of three communication signalsmultiplexed into three separate channels. The three signals would betransmitted into, or injected into, a communications system, for examplebut not limited to, a DSL subscriber loop. The vertical axis of thespectra is the signal strength or amplitude measured in dB, where dB iscommonly known in the art as decibels (dB). The horizontal axis of thespectra is signal frequency measured in Hertz (Hz). The same axisdefinitions will apply to FIG. 3B through FIG. 3E.

[0045] In FIG. 3A, the analog voice communication signal occupies theplain old telephone system (POTS) channel 34. As is well known in theart, the POTS channel typically occupies a bandwidth from about 0 to 4KHz. Two additional channels may be used in the DSL industry to transmitdigital data. In this embodiment of the DSL system, channel A 36occupies a bandwidth of 30 KHz to F1 KHz, and channel B 38 occupies abandwidth of F2 KHz to F3 KHz. Channel A 36 and channel B 38 eachcontain an ideal communication signal of a two channel DSL system. Thecommunication signals may be comprised of either analog or digital data.F1, F2 and F3 are communication bandwidth frequency boundaries of a PSDstandard 40 selected by the system design engineer. The 30 KHz lowerfrequency of the channel A 36 bandwidth is a typical value encounteredin the art, but which may be adjusted to a different value by the systemdesign engineer.

[0046] Shown in FIG. 3A with a dashed line is the power spectraldistribution (PSD) standard 40 for a channel A and channel B. A PSDstandard 40 defines the allowable PSD frequency range (bandwidth) andthe maximum signal strength (amplitude) for a communication channel atthe sending (transmitting) location. If the transmitted communicationsignal amplitude exceeds the PSD standard 40, then undesirableinterference or noise could be induced onto other subscriber loopssharing the same underground cable or overhead wire. If a transmittedcommunication signal amplitude is less that the PSD standard 40, thetransmitted communication signal is under-powered resulting in a lessthan optimal S/N ratio. If the bandwidth of a transmitted communicationsignal lies outside of the frequency boundaries of the PSD standard 40,then the transmitted communication signal may overlap onto and interferewith other communication channels. The transmitted communication signalsof channel A 36 and channel B 38 as shown in FIG. 3A are nearly ideal.That is, the two transmitted communication signals occupy the greatestregion of the PSD 40 standard without exceeding the amplitude andbandwidth limits as defined by the PSD standard 40.

[0047] Often, on a prior art two channel DSL system, a communicationsignal in one channel is traveling in the opposite direction of acommunication signal in the other channel. Direction of signal traveldepends upon the application of the DSL system user. As an illustrativeexample, the communication signal of channel A 36 could be transmittedat the CO digital equipment 21 (FIG. 1) into the subscriber loop 26 fortransmission to the CP digital equipment 52. Similarly, thecommunication signal of channel B 38 could be transmitted at the CPdigital equipment 52 into the subscriber loop 26 for transmission to theCO digital equipment 21. (For the remainder of the disclosure of thepreferred embodiment, for illustrative purposes only, the communicationsignal transmission location of channel A 36 will be designated as theCO 22 and the communication signal transmission location of channel B 38will be designated as the CP 24.) In actual practice of the prior art,signals may be transmitted from or received by both the CO digitalequipment 21 and the CP digital equipment 52.

[0048] Often, signal transmission direction in a channel changesdirection regularly, as in the POTS channel. For example, during atelephone voice conversation between two people over the PSTN, thespeaker determines the transmission location of the communication signaland the listener determines the location of the received signal. As aconversation proceeds between the two people, the direction of travel ofthe communication signal regularly changes depending upon which party isdoing the talking. Direction of travel of the communication signals of aDSL system can also be regularly changing.

[0049]FIG. 3B is illustrative of non-ideal communication signalamplitude spectra 132 which may be encountered with the prior art DSLtechnologies. The transmitted communication signal 136 of channel A isillustrated in FIG. 3B as degraded below the maximum signal strengthallowed by the PSD standard 40 due to effects of the actual impedance ofthe subscriber loop, the presence of bridged taps, wiring of thecustomer premises, and/or the various types of customer equipment aspreviously described in the Background section of this disclosure. Forfurther illustrative purposes, a part of the communication signalchannel B 138 has been degraded below the maximum signal strengthallowed by the PSD standard 40, while part of the communication signalchannel B 138 exceeds the maximum signal amplitude allowed by the PSDstandard 40. Also, the higher frequencies of communication signalchannel B 138 are greater than the high frequency (F3) bandwidth limitof the PSD standard 40 due to the reactive components of thetransmission system.

[0050]FIG. 3C is an enlarged view illustrating the non-idealcommunication signal amplitude spectra 238 of channel B (FIG. 3B) beforeprocessing by the SLD. Transmitting this non-ideal communication signalamplitude spectra 238 into a subscriber loop will cause a variety ofproblems, as previously discussed in the Background section. Thepreferred embodiment of the SLD 30 senses the direction of travel of acommunication signal and selects the signal if traveling in thetransmitting direction, defined as traveling in a direction out to thesubscriber loop. Once a communication signal has been selected, the SLD30 would continuously and automatically amplify a digital signal totransform the communication signal into a transformed communicationsignal such that the transformed communication signal conforms to apredefined specification. This specification would not be greater thanthe maximum amplitude allowed by the PSD standard 40. Here, in thisillustrative example, the lower frequency portion 238 a of the non-idealcommunication signal amplitude spectra 238 exceeds the maximum amplitudeof the PSD standard 40. If the communication signal portion 238 a isinjected (transmitted) into the subscriber loop, undesirableinterference could be induced in adjacent subscriber loops, aspreviously described in the Background section. That portion of thecommunication signal 238 a would be reduced (attenuated) by thepreferred embodiment of the SLD 30 to an amplitude value in closeproximity to the maximum amplitude of the PSD standard 40, as shown bythe transformed communication signal 338. Here, close proximity can bedefined as the amplitude of the transformed communication signal 338being below, at, or above the PSD standard 40, or another predefinedstandard, such that the error (difference) between the PSD standard 40and the transformed communication signal 338 is within some predefineddifference threshold.

[0051] Here, in the illustrative example of FIG. 3C, the mid-rangeportion 238 b of the non-ideal communication signal amplitude spectra238 is less than the maximum amplitude of the PSD standard 40. If thecommunication signal portion 238 b is transmitted into the subscriberloop, the S/N ratio will not be maximized, as previously discussed inthe Background section. The mid-range portion 238b of the non-idealcommunication signal amplitude spectra 238, which is below the maximumamplitude of the PSD standard 40, would be increased (amplified) by thepreferred embodiment of the SLD 30 to a value in close proximity to themaximum amplitude of the PSD standard 40, as shown by the transformedcommunication signal 338.

[0052] Another embodiment of the SLD 30 may have the additional featureof providing for frequency modulation, frequency shifting, or filteringa non-ideal communication signal to conform the transformedcommunication signal to a predefined frequency band specification thatis within the frequency bandwidth limits specified by the PSD standard40. As shown in the illustrative example of FIG. 3C, the highestfrequency portion 238 c of the non-ideal communication signal amplitudespectra 238 exceeds the high frequency limit F3 of the PSD standard 40.If the communication signal portion 238 c is transmitted into thesubscriber loop, undesirable interference could be induced in adjacentDSL channels, as previously described in the Background section. Thisembodiment of SLD 30 would frequency shift or filter the non-idealcommunication signal amplitude spectra 238 to fit within the frequencyboundaries of the PSD standard 40, as shown by the transformedcommunication signal 338.

[0053]FIG. 3D depicts an illustrative non-ideal communication signalamplitude spectra 438 before processing of a DSL channel. Anotherembodiment of the SLD 30 acts upon the non-ideal communication signalamplitude spectra 438 to conform the non-ideal communication signalamplitude spectra 438 to a predefined specification which is equal to apercentage of the PSD standard 40, as shown by the transformedcommunication signal 538. For illustrative purposes, FIG. 3D shows thetransformed communication signal 538 to be approximately 85 percent ofthe PSD standard 40. The SLD 30 continuously and automaticallydetermines the amount of amplification at any specific frequency of thenon-ideal communication signal amplitude spectra 438 and selects thedegree of amplification necessary to conform the non-ideal communicationsignal amplitude spectra 438 to the predefined specification of the PSDstandard 40. For example, the degree of amplification of the lowerfrequencies of the non-ideal communication signal 438 is seen to beabout ten to fifty percent. The degree of amplification of the higherfrequencies of the non-ideal communication signal 438 is seen to be asgreat as five hundred percent.

[0054]FIG. 3E depicts an illustrative non-ideal communication signalamplitude spectra 438 before processing by the SLD 30. Anotherembodiment of the SLD 30 modifies a non-ideal communication signalamplitude spectra 438 by simply amplifying the non-ideal communicationsignal amplitude spectra 438 by some fixed amount as determined by thepredefined specification, as shown by the transformed communicationsignal 638. For illustrative purposes, FIG. 3E shows the fixed amount ofamplification applied to the non-ideal communication signal amplitudespectra 438 to be approximately thirty percent of the non-idealcommunication signal.

[0055]FIG. 1 shows an existing telephone central office 22 and thecustomer premises 24 without the SLD 30 (FIG. 2). Digital signaltransmission and signal receiving equipment is depicted as the COdigital equipment 21 and the CP digital equipment 52. FIG. 4 shows amore detailed telephone system with installation of a telephony systemembodiment of the SLD 30. One skilled in the art will realize that thetelephone system illustrated in FIG. 4 can be replaced with other typesof communication systems where transmit signal processing by the SLDwould be beneficial. Other types of communication systems could include,but are not limited to, private telephony systems, coaxial cablesystems, fiber optic systems, microwave systems or radio communicationsystems.

[0056]FIG. 4 is now described in greater detail. Three communicationequipment components of the telephony system CO 22 are shown, thetelephony switching unit 28, digital equipment 21 and the SLD 30 a.(More communication equipment components, unrelated to the operation ofthe SLD 30 a, would likely be located at the telephone company CO 22,but are not shown in FIG. 4.) Three communication equipment componentsof the customer premises 24 are shown, a telephone 54, the SLD 30 b, andthe CP digital equipment 52. Examples of the CP digital equipment 52could be, but are not limited to, a computer, or a televisionset-top-box. For illustrative purposes for the preferred embodiment ofthis SLD, and as previously noted during the discussion of FIG. 3A, thecommunication signal transmission location of channel A 36 of the DSLsystem will be designated as the CO 22 and the communication signaltransmission location of channel B 38 will be designated as the CP 24.One skilled in the art will realize that the transmission location ofthe communication signals could be at either, or both, the CO 22 or theCP 24. Also, one skilled in the art will realize that any data channelcould be applicable to the illustrative example of FIG. 4 and to theapplication of the SLD.

[0057] When a communication signal is transmitted from the CO 22 to theCP 24 over channel A, the transmitted communication signal may not beideal (channel A 136 of FIG. 3B). The preferred embodiment of the SLD 30a, located at the CO 22, will continuously and automatically transform(amplify, attenuate and/or frequency modulate) a communication signalfrom the CO digital equipment 21 to conform to a predefinedspecification which does not exceed the signal strength or the frequencybandwidth of the PSD standard 40 (channel A 36 of FIG. 3A). The SLD thentransmits the transformed communication signal of channel A onto the DSL226 for transmission to the CP 24. When the communication signal isreceived at the CP 24, then becoming the receive signal, the receivesignal is delivered to the CP digital equipment 52. One skilled in theart will realize that the receive signal will pass through the SLD 30 bunaffected, or entirely bypass the SLD 30 b, depending upon the actualcircuitry configuration of the digital signal processing equipment. Thatis, the preferred embodiment of the SLD will sense the direction oftravel of the communication signal and selectively operate only in thecommunication signal transmission direction.

[0058] Similarly, when a communication signal is transmitted from the CP24 to the CO 22 over the channel B, the communication signal may not beideal (channel B 138 of FIG. 3B). The preferred embodiment of the SLD 30b, located at the CP 24, will transform (amplify, attenuate and/orfrequency modulate) a communication signal from the CP digital equipment52 to conform to a predefined specification which does not exceed thesignal strength or the frequency bandwidth of the PSD standard 40(channel B 38 of FIG. 3A, or channel B 338 of FIG. 3C). The SLD 30 bthen transmits the transformed communication signal of channel B ontothe DSL 226 for transmission to the CO 22. When the communication signalis received at the CO, then becoming a receive signal, the receivesignal is delivered to the CO digital equipment 21. One skilled in theart will realize that the receive signal will pass through the SLD 30 aunaffected, or entirely bypass the SLD 30 a, depending upon the actualcircuitry configuration of the digital signal processing equipment.

[0059] As shown in FIG. 4, and which is well known by those skilled inthe art, the analog telephony signal transmitted on the POTS channel 34(FIGS. 3A and 3B) between the CO telephony switching unit 28 and thetelephone 54 over the DSL 226 is transmitted without interacting withthe DSL, 30 a or 30 b, which is transmitting over channels A and B.

[0060]FIG. 5A is a block diagram showing two of the components of thepreferred embodiment of the SLD 30, a transmit signal equalizer 60 and acurrent driver 62. The transmit signal equalizer 60 detects the incomingcommunication signal (not shown), and transforms (amplify, attenuateand/or frequency shift) the communication signal to conform to apredefined specification. The current driver 62 then transmits thetransformed communication signal into the communication connection 126.One skilled in the art will recognize that the degree of communicationsignal distortion and the amount of amplification and frequencymodulation required to transform the communication signal will dictatethe complexity of the transmit signal equalizer 60. FIG. 5B is avariation of the SLD 30 wherein a voltage driver 64 is used to injectthe transformed communication signal into the communication connection126.

[0061]FIG. 6 shows two enhancements of the SLD 30 of FIG. 5A. The firstenhancement is a voltage feedback loop wherein an amplifier 66 providessignal feedback to the transmit signal equalizer 60. The feedback loopdetects a communication signal that may not be ideal (Channel B 138 ofFIG. 3B) and provides for the continuous and automatic adjustment of thecommunication signal after the current driver 62 injects the transformedcommunication signal into the subscriber loop. The SLD 30 has thecapability to provide a transformed communication signal PSD that isideal regardless of the transmission channel impedance. Also, the SLD 30has the capability to provide a transformed communication signal PSDthat is ideal regardless of other multipoint transceivers. Once the SLD30 transmit signal equalizer 60 has been calibrated for a particular DSLcircuit, there is no need for continuing recalibration under practicalapplications. Here in FIG. 6, the subscriber loop is shown as a twistedpair copper wire local loop 326 of a telephony system or a DSL systemconsisting of a Tip 70 line and a Ring 72 line. The twisted pair copperwire local loop 326 is referenced in FIG. 1 as the telephony systemsubscriber loop 26 and in FIG. 4 as the DSL 226. As is well known bythose skilled in the art, all of the above expressions describingtelephony and DSL communication systems may be equivalent.

[0062] The second enhancement of the SLD 30 shown in FIG. 6 is theaddition of a parallel resistor 68 of some finite impedance. The SLD 30enjoys an infinite input impedance, often defined in the prior art asRR. Note especially that with the SLD 30, an infinite input impedance RRis true for all frequencies. An infinite input impedance of the SLD 30in the POTS band is desirable, as there would be no loading of the POTSband. And, although tradition of the prior art implies that forpractical applications the terminating impedance of a transmission lineshould be assumed to be the “characteristic impedance” of thetransmission line, one skilled in the art will realize that this is anincorrect conclusion based on “maximizing power transfer.” In actuality,the ideal signal transmission optimization technique is to maximize thereceive signal level as long as loss vs. frequency is within thetolerances of the receive signal equipment (can be read with acceptablebit error tolerances) and potential signal reflection on thetransmission line is suitable. Although tradition of the existing priorart indicates the frequency band above 25 kHz should be terminated from100 ohms to 135 ohms, empirical tests show that that termination at 1000ohms or higher, or even at an infinite impedance, would provide forsuperior voltage signal transmission. One skilled in the art willrecognize that the simple addition of a parallel resister 68 shown inFIG. 6 can enable the design engineer to set the transmission systemterminating impedance to any desired value without compromising theother attributes of the subscriber loop or the SLD 30.

[0063] Another benefit is provided by the infinite input impedance ofthe SLD 30. “Splitter-less” DSL technologies, well known in the art,require a subscriber loop transmission system with a relatively low RTand RR in the DSL frequency bands while having a relatively high RR inthe POTS frequency band. For example, a desirable DSL transmissionsystem RT and RR would be 100 ohms at 26 kHz and above, and for the POTSperhaps 1200 ohms at 4 kHz and below. This desirable DSL transmissionsystem is very difficult, and perhaps impossible, to achieve with theprior art. The SLD 30 provides a way to implement specified impedanceson a DSL system which provides for desirable impedances on both a POTSchannel and splitter-less DSL channels. Also, the infinite inputimpedance of the SLD 30 minimizes the adverse affects of the POTSon-hook/off-hook transition on the DSL channel.

[0064] Yet another practical benefit from the SLD 30 is optimizing a DSLtransmission system when two or more transceivers are placed at one orboth ends of the subscriber loop, as in multipoint communication. Thetransmitted communication signal amplitude would, in the absence of theSLD 30, be significantly reduced due to the lowered net load impedanceseen by that transmitter. For two transceivers, the transmittedcommunication signal could be reduced by as much as 4 dB. Similarly, theeffective RR now becomes the parallel combination of the RR of the twotransceivers, and the receive signal is reduced. Thus, the SLD providesfor a transmitted communication signal which is not affected by thepresence of multipoint operation, thereby optimizing the receive signal.

[0065] The inclusion of an SLD 30 into a larger system may be consideredas an improvement to the larger system. When an SLD is incorporated intoa CO 22, as shown in FIG. 7A, the CO 22 is improved in that the CO 22may now transmit transformed communication signals from CO 22 digitalequipment 21 which have been modified to conform to a predefinedspecification. Similarly, an SLD can be incorporated into a PBX 23 as animprovement, as shown in FIG. 7B. In both the CO 22 and the PBX 23, atleast one SLD may be installed at the CO 22 or PBX 23, with one SLD 30being ultimately located at some point between the digital equipment 21or 121 and communication system transmission line, such as, but notlimited to, a DSL 226.

[0066] The SLD 30 may be considered as an improvement to a transmitter130 system. The SLD 30, when incorporated into the transmitter 130,would transform communication signals to conform to a predefinedspecification. A transmitter 130 with an SLD 30 is shown in FIG. 8. TheSLD 30 is ultimately connected to a communication system transmissionline, such as, but not limited to, a DSL 226.

[0067] It should be emphasized that the above-described embodiments ofthe present invention, particularly, and “preferred” embodiments orconfigurations, are merely possible examples of implementation, merelyset forth for a clear understanding of the principles of the invention.Many variations and modifications may be made to the above-describedembodiment(s) of the invention without departing substantially form thespirit and principles of the invention. All such modifications andvariations are intended to be included herein within the scope of thepresent invention.

What is claimed is:
 1. A subscriber line driver (SLD) for transformingamplitude characteristics of a communication signal, comprising: meansfor detecting amplitude of a first portion of the communication signal,the amplitude greater than a predefined specification; means fordetecting amplitude of a second portion of the communication signal, theamplitude less than the predefined specification; means for decreasingthe amplitude of the first portion so that the decreased amplitude is inclose proximity to the predefined specification; and means forincreasing the amplitude of the second portion so that the increasedamplitude is in close proximity to the predefined specification, therebyresulting in a transformed communication signal.
 2. The SLD of claim 1,further comprising a transmitting means for transmitting saidtransformed communication signal onto a communication medium pursuant tothe predefined specification.
 3. The SLD of claim 1, wherein saidpredefined specification is a power spectral distribution (PSD)standard.
 4. The SLD of claim 1, wherein said predefined specificationis a fraction of a power spectral distribution (PSD) standard.
 5. TheSLD of claim 1, wherein the communication system is a digital subscriberloop (DSL).
 6. A subscriber line driver (SLD) for transforming amplitudecharacteristics of a communication signal, comprising a signal equalizerconfigured to decrease amplitude of a first portion of the communicationsignal, the first portion having a first amplitude greater than apredefined specification, so that the first adjusted amplitude is inclose proximity to the predefined specification, and further configuredto increase amplitude of a second portion of the communication signal,the second portion having a second amplitude less than the predefinedspecification, so that the adjusted second amplitude of thecommunication signal is in close proximity to the predefinedspecification, thereby resulting in a transformed communication signal.7. The SLD of claim 6, further comprising a current driver, the currentdriver configured to receive communications from the signal equalizer sothat said current driver injects the transformed communication signalinto the communication system.
 8. The SLD of claim 6, further comprisinga voltage driver, the voltage driver configured to receivecommunications from the signal equalizer so that said voltage driverinjects the transformed communication signal into the communicationsystem.
 9. The SLD of claim 6, further comprising a voltage feedbackamplifier circuit wherein the inputs to the voltage feedback amplifiercircuit are connected to a tip wire and a ring wire of a telephonysystem, and the output of the voltage feedback amplifier circuit isconnected as an input to the signal equalizer.
 10. The SLD of claim 1,further comprising a means for sensing the direction of travel of saidcommunication signal and a means for selectively operating in a transmitsignal direction when the communication signal is traveling in adirection out to a communication transmission system.
 11. A method fortransforming characteristics of a communication signal, comprising:decreasing amplitude of a first detected portion of the communicationsignal so that the decreased amplitude is in close proximity to apredefined specification; and increasing amplitude of a second portionof the communication signal so that the increased amplitude is in closeproximity to the predefined specification, thereby resulting in atransformed communication signal.
 12. The method of claim 11, furthercomprising: detecting amplitude of the first portion of thecommunication signal, the first portion having a first amplitude greaterthan the predefined specification; and detecting amplitude of the secondportion of the communication signal, the second portion having a secondamplitude less than the predefined specification.
 13. The method ofclaim 11, wherein the decreasing further comprises decreasing amplitudeof the first portion to within a predefined difference threshold. 14.The method of claim 11, wherein the increasing further comprisesincreasing amplitude of the second portion to within a predefineddifference threshold.
 15. The method of claim 11, further comprising:sensing direction of travel of the communication signal; and selectivelyoperating in a transmit signal direction when the communication signalis traveling in a direction out to a communication transmission system.16. The method of claim 11, wherein said predefined specification is apower spectral distribution (PSD) standard.
 17. The method of claim 11,wherein the communication system is a digital subscriber loop (DSL).